1. Field of the Invention
The invention relates to a method of making porous lead titanate (PT) or lead zirconate (PZT) ceramics of a Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 compound where x=0 to 0.9, by using a sol-gel-process.
Solids and gases have noticeably different acoustic impedances. For that reason, conventional ultrasonic sensors and actuators made of dense ceramics and ceramic-polymer composites are suited to a limited extent only for transmitting signals in gaseous media. By comparison, porous piezo-active materials in their density and sound velocity and, hence, in their acoustic impedance are better adapted to air space by an order of magnitude.
2. The State of the Art
Two methods are known for producing such porous substances, viz., a slip casting method and an aerogel method. In the slip casting method, PZT powder is mixed with methylethyl cellulose or other binders and is isostatically cold pressed. Following a temperature treatment of up to 1,100.degree. C., ceramic materials are obtained having porosities of about 50% (G. Galassi et al., Fourth Euro Ceramics 5 (1995), 25-32). An aerogel process is described in European Patent specification EP 0,470,898 Al, in which a ceramic gel (PZT) is produced in the presence of a solvent from lead carboxylates and titanium/zirconium alkoxides by a sol-gel process. By applying pressures and temperatures which convert the solvent into a supercritical state, a material is obtained in a porous condition as an aerogel. By calcination, crystalline ceramics of high porosity are derived therefrom. It has, however, been shown that lead carboxylates, such as, e.g., lead acetate trihydrate (Pb(OAc).sub.2 3 H.sub.2 O is solvated to a very limited extent only in the mentioned solvents such as methanol, ethanol, isopropanol or acetone by which the supercritical drying is then carried out. Because of the low lead and titanium/zirconium concentration it is possible to produce wet gels with difficulty only. These, however, have so filiform a skeleton structure that at supercritical drying they are almost entirely destroyed in their own solvent. Monoliths cannot be produced by the method described in EP 0,470,898 A1.